The invention relates to the tread of a tire designed more particularly to run on snowy, icy or wet surfaces.
Such a tread is conventionally provided with elements in relief of the rib or block type, separated from one another in the circumferential direction and/or in the transverse direction by transverse and/or circumferential grooves. The transverse direction is defined as a direction parallel to the axis of rotation of the tire; the transverse direction belongs to a meridian plane. The circumferential direction is perpendicular to the transverse direction and corresponds to the longitudinal direction. The elements in relief are defined by a surface forming part of the surface of the tread and at least one lateral face perpendicular to or oblique relative to the running surface. More generally, the elements in relief comprise at least two faces, the instance of a single face corresponding to elements in relief of rounded section.
Such a tread generally further comprises incisions or slits, the widths of which are other than zero but far less than those of the above-mentioned grooves. By making a plurality of cuts opening onto the running surface, a plurality of rubber ridges is created for cutting through the layer of water which may be present on the road, in such a way as to keep the tire in contact with the ground and to create cavities optionally forming ducts intended to collect and remove the water present in the contact zone between the tire and the road once they are disposed in such a way as to open outside the contact zone.
Numerous types of incision have already been proposed with a view to improving tire grip on the surfaces in question.
French Patent 2,418,719 (corresponding to U.S. Pat. No. 4,298,046) in particular describes incisions which may be normal to the surface of the tread or inclined relative to the direction perpendicular to said surface.
Document FR 791 250 describes incisions following a wavy course over the surface of the tread.
However, it would appear that increasing the number of cuts may lead to a reduction in the rigidity of the tread, which has an unfavorable impact on the performance of the tire, not to say on grip performance. The rigidity of the tread is understood to mean the rigidity of the tread under the combined effect of compressive stresses and shearing forces in the area in contact with the road. It is known, in effect, that to improve the grip of a tire on the surfaces in question, it is necessary to optimize the contact surface, that is to say the latter should be as large as possible. For this purpose, it is necessary to provide flexibility of contact with the ground in a radial direction. On the other hand, it would appear that the loss of rigidity, in the longitudinal direction and optionally in the transverse direction, due to the presence of a large number of cuts has a tendency to reduce this contact surface.
In effect, tipping-over of the elements of the tread pattern is noted, due to shear for example as a result of driving torque or braking torque. This tipping-over causes a reduction in the contact surface, which leads to a reduction in grip.
To remedy this problem associated with loss of rigidity in the longitudinal and possibly transverse directions, it has already been proposed to provide blades of the self-locking type, that is to say blades which rest against one another under radial compression such that longitudinal and transverse rigidities are increased.
Document EP 0 282 765 (corresponding to U.S. Pat. No. 4,794,965) describes in particular incisions which exhibit broken or wavy lines over the entire depth thereof. When subject to radial compression, the walls of the incisions move closer to one another, so promoting an increase in longitudinal rigidity, the overlapping of said walls resulting in a self-locking effect.
French Patent 2,722,144 (corresponding to U.S. Pat. No. 5,783,002) also describes incisions, the walls of which each comprise zones in relief formed by protrusions and cavities designed as before to overlap under radial compression. Overlapping then results in an increase in longitudinal and transverse rigidities.
To optimize the contact surface, it is additionally known to use low modulus rubber mixes, so as to reduce radial rigidity. However, the industrial methods used in the manufacture of semi-finished products limit the choice of rubber mix moduli and do not allow the use of an optimum modulus as far as compressibility is concerned.